While most evolutionary researchers agree that intergroup competition, especially in the form of violent conflict, was probably part of life in ancient hunter-gatherer societies, there are two main alternative views regarding how this competition influenced our genetic evolution. The canonical view, staunchly defended by the psychologist Steven Pinker, is that intergroup competition plays no role in shaping either genetic or cultural evolution. An alternative view, recently reenergized, is that intergroup competition shaped not cultural evolution - as I argue - but genetic evolution. Under this view, warfare and differential extinction drove genetic evolution and shaped human nature directly (Haidt 2012, Wilson 2012, Wilson and Wilson 2007, Bowles 2006). The first view is contradicted by evidence showing that intergroup competition leads to the differential spread of certain cultural traits, including both social norms and technologies. Intergroup competition also helps account for the intricate and subtle institutions we commonly observe across the diverse societies that expand and sustain cooperation. Adherents to the canonical view are stuck arguing that yes, intergroup competition was common, but no, it somehow never influenced which social norms or practices survived, were copied, and spread. You've now only seen the tip of an iceberg of the evidence showing the importance of intergroup competition for cultural evolution (Richerson et al.. forthcoming). For Pinker's view on group selection, see http://edge.org/conversation/the-false-allure-of-group-selection. However, be sure to read my commentary on Pinker's piece at the same site.

The other view, that intergroup competition has directly shaped human genetic evolution, need not conflict with what I'm focused on here. However, there are a couple of reasons to suspect that the direct role of intergroup competition on genetic evolution is, at least, secondary to the process I'm describing, and possibly trivial. Here's the key: for intergroup competition to have any effect on evolutionary processes, be they cultural or genetic, groups have to remain relatively distinct along whatever dimensions are providing some groups with competitive advantages over others groups. For social norms, this is easy to see. If I move into your group from another group, my kids and I have to adopt your kinship and marriage norms. If we don't, my kids either won't have any relationships (which govern helping, food sharing, sex, trade, etc.) or they will be doing all the wrong things (norm violations). They might, for example, repeatedly make Kula's mistake and violate an incest taboo by sitting near the wrong girl or boy, which will get them sanctioned in some way. However, for genes, if people from different groups have sex, the relevant genetic differences between the groups will quickly go away. Either they initially advantaged groups will get the "bad" genes from the disadvantaged groups, or the disadvantaged groups will get the "good" genes. This genetic mixing means the groups will become increasingly indistinct. The point is, cultural evolution can sustain differences between groups in a manner that genetic evolution cannot. Exacerbating this genetic mixing is the fact that human-style intergroup competition often increases the flow of genes between groups. Victorious groups in warfare frequently take the younger women and girls from the defeated groups as "wives" - in fact, access to "wives" is often the explicit reason why men from one group attack another. This creates a big inflow of genes from the losers to the winners. Or, in the absence of violence, it's still the case that men from more successful groups look for, and often find, their future wives (or short-term mates) in less successful groups. This again causes genes to flow rapidly into the more successful groups - which will wipe out the genetic differences between the groups. The couple's children might adopt all of their father's social norms, by living in his community, but no matter what, they retain half of their mother's genes. This, and other forms of differential migration, deplete genetic differences among groups while not reducing cultural differences. Data on genes and culture from the modern world confirm these stark differences, with many genetically indistinguishable groups remaining culturally quite different; see Bell, Richerson, and McElreath 2009 for analyses of genetic versus cultural variation. More generally, see Henrich 2004a, N. Henrich and Henrich 2007, and Boyd, Richerson, and Henrich 2011b.

Beyond this, our species' capacity for large-scale cooperation is tightly hinged to the presence of culturally evolved reputational and sanctioning systems, and on internalized social norms. Thus, the psychological evidence regarding human sociality and morality is most consistent with innate mechanisms adopted to a culturally constructed world (see chapter 11). It's difficult to square this empirical evidence with either of the alternative views described above.